1. Field of the Invention
This invention relates generally to mooring systems for floating production, storage and offloading vessels (FPSO). In particular the invention pertains to a swivel arrangement by which risers from subsea wells are rotatively coupled to outlet pipes which run to storage holds on the vessel. Still more particularly, the invention relates to a mechanical arrangement by which outer housings of a swivel stack are coupled together to allow them to rotate with the vessel about a non-rotatable inner housing.
2. Description of the Prior Art
Swivel stacks are known for rotative coupling of risers to outlet pipes on the vessel. A typical swivel stack includes multiple swivels stacked on top of one another with the inner core of each swivel each secured together to form a swivel core stack which is stationarily carried by a non-rotatable turret structure. The turret structure is maintained in a substantially non-rotatable (or "geo-stationary") state by anchor legs which extend to the sea floor. The anchor legs may be connected directly to the turret, as in the case of a permanently moored system, or to a disconnectable spider buoy as in the case of a disconnectable system.
The vessel is rotatatively coupled to the turret by a bearing arrangement and is designed and arranged to weathervane about the turret due to environmental forces on the vessel which create an effective torque on the vessel about the center line of the turret.
Hydrocarbon risers extend from subsea wells or manifolds, run via the interior of the turret, and are terminated on a manifold deck carried by the turret. Prior art arrangements have placed the swivel stack above the manifold deck so that manifold pipes may easily enter the stationary core of the swivel stack and so that a torque arm from the vessel may be easily connected to each outer housing of each swivel in the stack. Such prior art arrangements have created stack heights which extend a great distance above the top of the turret.